Allelopathy:

Some plants prevent other plants from growing too close. They do this by making and releasing special chemicals called allelochemicals. Sweet potatoes contain many allelochemicals. ARS scientists are looking for ways to use these substances as natural weed killers.

There are several types of chemical allelopathy. In one type, the plants release allelochemicals from their roots. Nearby plants absorb these chemicals from the soil and die. Other kinds of allelochemicals can slow down or stop a plant's process photosynthesis--its chemical machinery for converting the sun's energy into food.